Processes for dyeing polyester-cotton mixed fibres and comprising more than one step, where the blend components are dyed in separate steps with different dyes, are already known.
These processes exhibit serious drawbacks, such as complex realization, possibility of reciprocal spotting, necessity of using high amounts of dyes.
One of these dyeing processes is described, for example, in U.S. Pat. No. 3,313,590.
Processes for dyeing polyester-cotton mixed fibres, utilizing the capability of the cellulose material of being swollen by water are known too.
It is known in fact that cotton fibres and cellulose materials are quickly swellable with water, and that swelling is accelerated by the action of wetting agents and/or of heat.
Such swollen materials have modified physical and mechanical properties, i.e. an opener structure, a lower mechanical resistance, a higher flexibility, wherefore they are more easily penetrated by reactive agents and/or dyes.
It is also known that cotton can be treated with water-soluble dyes when it is swollen with water containing high-boiling water-soluble solvents having a delimited molecular weight (polyoxyethylene glycol). This process has been mentioned by E. I. Valko and K. M. Limdi in "Textile Research Journal" 32, 331-7 (1962).
According to the Authors such method is applicable also for introducing into cotton materials insoluble in water, but soluble in the above-mentioned water-soluble high-boiling solvents.
This method has been followed by J. Blackwell et al., who have described, in U.S. Pat. No. 3,706,525, the application, in an only step, of water-insoluble dyes preformed on swellable cellulose and/or blends thereof with synthetic fibres such as, for instance, polyester and polyamide fibres, by using high-boiling solvents such as polyols and ethers and esters thereof, which keep cellulose in the swollen state also after evaporation of water during the intermediate drying step of the thermosol process.
The dyes to be employed according to the last described process shall meet the following important requirements:
good affinity for polyester fibres, PA1 good affinity for cellulose fibres swellable by the water-solvent mixture, PA1 low tendency to sublimation at the application temperature, PA1 capability of providing uniform dyeings on both fibres, PA1 capability of providing dyeings and printings resistant to sunlight, to rubbing, to washing.